Dynamically access property with typecasting in TypeScript with generic

Question

I have following simplified function that accept generic.

function filter<T>(data: T[]) {
    
    const filter = { field: 'name', type: 'string', condition: 'ct' }

    data.forEach((value) => {
    
        type DataPropertyValue = keyof typeof value
    
        // Following does not work 
        // const dynamicValue = (value[filter.field as DataPropertyValue] as String).toLowerCase() --- This does not work
        // const dynamicValue = value[filter.field as DataPropertyValue].toLowerCase() --- This does not work
    
        // But this works but I have to re-instantiate String here
        const dynamicValue = String(value[filter.field as DataPropertyValue]).toLowerCase()
        console.log(dynamicValue)
    })
}

This works but as you see I have to recreate instance of String to operate on the value. Is there anyway to do this without re-instantiating String again.

Answer 1

The problem is that value is of type T, which could be anything. Which means that the property you are trying to look up may not actually have a string type.

For example:

filter([{ name: true }])

Here T would be of type { name: boolean } and toLowerCase called on a boolean would throw an exception.

---

You could fix this by adding code that checks to make sure the value is the right runtime type before proceeding.

That might look something like this:

function filter<T>(data: T[]) {
    
    const filter = { field: 'name', type: 'string', condition: 'ct' }

    data.forEach((value) => {
        const dynamicValue = filter.field in value ? value[filter.field as keyof T] : undefined
        if (typeof dynamicValue === 'string') {
            console.log(dynamicValue.toLowerCase())
        } else {
            console.log('unsupported property type')
        }
    })
}

filter([{ name: true }]) // logs "unsupported property type"
filter([{ name: 'FOO' }]) // logs "foo"

I'm not sure if that's exactly what you after, but I think it's closer.

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